In large environments you might need to think about the IO throughput on your DB and how this compare on physical vs virtual machines. I would assume a physical box with seperate pyysical disks for OS and Db would be the most performance benificial configuration.

I am using a VM for DB SQL 2008, win 2k8 64 bit. We had to put 24GB of RAM and use 8 CPU's with VMAX Storage 2 TB and that seems to run well. We are monitoring way over 1500 servers. Just make sure to use non contendeing IO's and spread them out well on the LUNS.